Hull speed

[Grehan]

Using the formulae available on www.johnsboatstuff.com (jolly interesting if nothing else) I have determined that Grehan's hull speed is 6knots (through the water).
I understand about how the boat sits in the water profile when travelling through it and hence the 'ideal' maximum speed before more velocity becomes inefficient.
[well at least I hope I do . . .]

Ah, but what happens when going faster than 6knots?
Is that just kind of 'inefficient' (ie there's a lot of effort for decreasing result) and also that the boat tends to bury her nose into the 'hole' in the water? Or something else / something more?

Should the sails be adjusted (reefed) to reduce any excess speed effectively to 6knots?

Have I got the wrong end of the stick somewhere?
I'm not exactly losing sleep over this, but some illumination/explanation would be gratefully received. Merci beaucoup.

 

[DoctorD]

A boat is mimited to its hull speed which is dependent on waterline length, UNLESS it is planing(dinghy) or surfing (big boat). i.e. my boat has a hull speed of around 8 knots (36 foot sailboat) and the engine cannot push it faster than this speed - if I increase revs above a certain point I just burn more fuel. But when I am on a broad reach in a lot of wind I can go over 10 knots at times - this is becuase the boat is lifting right out of the water for short periods of time as it surfs the top of the waves.

Surfing is very efficient.

Graham

 

[ParaHandy]

Not sure that's correct...as she leans over, her water line length increases which is where the extra speed comes from?

 

[jollyjacktar]

What about a catamaran?

Cats seem to defy thes rules, how come? Do two hulls double the speed?

 

[Chris_Stannard]

Don't loose any sleep. In the first place it is only an indication. I had a Sigma 362, LWL 28 feet gives a hull speed before it tries to climb out of the trough of 7.1 knots. As the boat heeled and sat down the LWL increased but of course could not increase beyond 36 feet = 8 knots. Beyond those sppeds the power required to increase the speed increases dramatically, but if the power is there the speed will increase. In strong winds and with strong nerves when racing we often got over 9 knots on a reach. On a run the spinaker would tend to lift the bow making planing more of a possiblity. It is why you see all the guys standing at the back end of a racing boat going down wind. You then get a lift up component from the water which means that planing becomes easier.

Obviously the weight and hull form of the boat will affect the power needed for any speed, so a heavy boat would tend to need more power than a light boat and a clean hull will not need so much power as a foul hull, you can see this by setting the engine revs at say 2500 and seeing what you log through the water before and after you antifoul. It is also the reason it is a good idea to avoid all imperfections in the antifoul, it adds to the power requirement and will therefore limit your speed.

In short, don't worry about it just enjoy your sailing

Chris Stannard

 

[claymore]

Re: What about a catamaran?

No, cats get their efficiency from having less hull in the water which means less drag.
I'm not sure how the hull speed formula fits with cats

 

[bedouin]

Re: What about a catamaran?

IIRC - the conventional hull-speed formula doesn't hold for narrow-hulled boats (LWL > 10x beam ?), which is why cats can do so well

 

[Boathook]

Re: What about a catamaran?

I always thought that the formula was the same. Don't forget that Cats hulls are narrow for the length and therefore should have a higher speed. Also, Cats do not heel as such, so you do not 9hopefully) lose power that way like a mono.

 

[Stemar]

Re: What about a catamaran?

Hull speed applies to cats as well. as with anything that floats, If they go significantly faster under their own power, it's because they're planing, the same as a power boat. A 25ft waterline boat will have a hull speed of about 6 1/2 knots. To get above this speed takes a lot of power to climb up the bow wave, but when it gets on top it planes along the surface and needs a lot less. to keep it there.

 

[claymore]

Re: What about a catamaran?

Yes I understand that point - my question - which I may not have worded very well - was can you apply the same formula for hull speed to a cat (or multihull) as for a monohull. I'm not very good at physics but it seems that there should be a different formula as there is less drag - via a reduced wetted surface area - so the forces are different therefore the formula ought to be different?

 

[ParaHandy]

Re: What about a catamaran?

Think it's something to do with the wave created by the hull. Something like wave period can't exceed length or could be complete bolx? Wave created by friction of wetted surface and if v. small surface = no wave?

Oh hell........

 

[Chris_Stannard]

Re: What about a catamaran?

As I understand it the hull speed rule applies to cats as to anything else. The difference with a cat is that it does not carry a bloody great lump of iron around. It depends for its stability on having two areas of flotation so that which ever way it heels the centre of bouyancy is well outside the sentre of gravity and you have a strong righting lever, until you go too far and tip it over.
Because the cat is not carrying ballast it has a much lower wetted area and therefore requires less power to make it go. It also the same proportion of power to go from non-planing to planing, but because it starts from a much lower base it it much easier to get out of the trough caused by your own hull form wave. Or perhaps I am talking through the top of my head


Chris Stannard

 

[ParaHandy]

Re: What about a catamaran?

Might not carry a lump of iron but the one I used to race did carry two largish bods. Kept getting beaten (thrashed more like) by 6 stone 13 yr olds. What you say all sounds right although you actually can not get them moving until the windward hull is just about out of the water although that might be because we were never too sure whether the hulls weren't cross-eyed. Unlike P Goss's affair, we at least had a bridle to tie the forward sections of the hulls to the forestay....

 

[Guest]

Hi,

Hull speed can certainly be a bit of a misnomer - it's difficult to get exactly unless you know lots about a particular boat, its hull, loading etc. In general, the speed of a boat compared against its size is measured in a value called the Froude number. This has the formula FROUDE NUMBER= VELOCITY/((9.81*L)^0.5), where L is the length of the waterline in metres and velocity is the boat speed in metres per second. A boat effectively reaches hull speed at a Froude number of 0.54, so your theoretical hull speed becomes VELOCITY=0.54*((9.81*L)^0.5). Divide velocity by 0.5148 to get the speed in knots. Above this point the resistance of the hull increases very quickly (double the total hull speed resistance one knot above hull speed is not unusual). Traditional yachts with a heavy build never stood a chance of developing enough power to get above it, but a modern, very light racing yacht might be capable of developing enough power downwind with a bit of surf to help, but the average cruising yacht will be very unlikely to truely plane, and certainly won't without a little bit of swell to nudge it along. The increase in resistance is due to 2 components of resistance called residual resistance and viscous pressure drag. The residual resistance around normal, curved hull forms is the dominant resistance around hull speed (about 90% of the total drag). This is caused by pressure changes around the hull underwater, which can be seen on the surface as waves. Its important to remember these pressure differentials exist under and around the hull as well. In certain hull forms, despite popular belief, these can cause enough suction to actually create a slight sinkage on the hull at speeds instead of lift, increasing resistance even more. The viscous pressure drag is caused by the entrained water behind the hull (particularly if a small bit of the transom is immersed as the beer gets passed to the cockpit), and is due to the internal visocity of the water overcoming other forces and dragging a small amount of water behind the boat, effectively adding to the displacement. It's s bit like dragging a heavy dinghy behind the boat when you get above hull speed. Cats can be a bit deceptive- they're only fast if they're light enough. If they are relatively light, then in general the finer hull results in less pressure changes and less residual resistance, hence more speed for a given power (the upright rigs are also more efficient). However, the interaction of the viscous drag can, if you're not careful, at some Froude numbers below 0.54, actually increase the resistance over monohulls, but that's getting into really complicated fluid dynamics and it's time for the pub...

Hope this helps.

 

[chippie]

Thankyou for your informative post. I sail a trimaran and have had it surfing on occasion. It is always important to make sure the transom doesnt drag by being too low.
My Question is: what are the influences on the two immersed hulls (main & leeward). Are there contradictory things happening given that they are of such different size and shape?
How do the froude numbers apply in this example?

 

[Guest]

An easier way to compute your hull speed..... multiply the square root of your waterline X 1.4.

My boat has a waterline of 27 so: 5.196 or 5.2 X 1.4 = 7.28 knots.

The rule is not cut in stone and on this side of the pond variations place the multiplier at anywhere between
1.34 to 1.4.